The Third Generation Partnership Project (3GPP) initiative “License Assisted Access” (LAA) intends to allow Long Term Evolution (LTE) equipment to also operate in the unlicensed 5 Giga Hertz (GHz) radio spectrum. The unlicensed 5 GHz spectrum is used as a complement to the licensed spectrum. Accordingly, devices connect in the licensed spectrum with a primary cell (PCell) and use carrier aggregation to benefit from additional transmission capacity in the unlicensed spectrum using secondary cell(s) (SCell). To reduce the changes required for aggregating licensed and unlicensed spectrum, the LTE frame timing in the primary cell is simultaneously used in the secondary cell.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum may be shared with other radios of similar or dissimilar wireless technologies, a so called listen-before-talk (LBT) method is applied. Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the Institute of Electrical and Electronics Engineers (IEEE) 802.11 Wireless Local Area Network (WLAN) standard, known under its marketing brand “Wi-Fi”.
The LBT procedure leads to uncertainty at the evolved Node B (eNB) base station regarding whether it will be able to transmit downlink (DL) subframe(s) or not. This leads to a corresponding uncertainty at the User Equipment (UE) as to if it actually has a subframe to decode or not. An analogous uncertainty exists in the uplink (UL) direction where the eNB is uncertain if the UEs scheduled on the SCell actually transmitted or not.
Long Term Evolution (LTE)
LTE uses Orthogonal Frequency-Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT) spread OFDM, also referred to as single-carrier (SC) Frequency-Division Multiple Access (FDMA) in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid with time on x-axis and frequency on the y-axis, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. The uplink subframe has the same subcarrier spacing as the downlink and the same number of SC-FDMA symbols in the time domain as OFDM symbols in the downlink.
In the time domain, LTE downlink transmissions are organized into radio frames of to milliseconds (ms), each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each symbol is approximately 71.4 microseconds (μs).
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about which terminals data is transmitted to and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of e.g. the control information. A normal subframe of a downlink system with CFI=3 OFDM symbols as a control region is illustrated in FIG. 1.
From the LTE standard specification Release 11 (Rel-11) onwards, above described resource assignments can also be scheduled on the enhanced Physical Downlink Control Channel (EPDCCH). For Rel-8 to Rel-10 only Physical Downlink Control Channel (PDCCH) is available.
The reference symbols shown in the above FIG. 1 are the cell specific reference symbols (CRS) and are used to support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes.
Physical Downlink Control Channel (PDCCH) and Enhanced PDCCH (EPDCCH)
The PDCCH/EPDCCH is used to carry downlink control information (DCI) such as scheduling decisions and power-control commands. More specifically, the DCI includes:                Downlink scheduling assignments, including Physical Downlink Shared Channel (PDSCH) resource indication, transport format, hybrid-Automatic Repeat Request (ARQ) information, and control information related to spatial multiplexing (if applicable). A downlink scheduling assignment also includes a command for power control of the Physical Uplink Control Channel (PUCCH) used for transmission of hybrid-ARQ acknowledgements in response to downlink scheduling assignments.        Uplink scheduling grants, including Physical Uplink Shared Channel (PUSCH) resource indication, transport format, and hybrid-ARQ-related information. An uplink scheduling grant also includes a command for power control of the PUSCH.        Power-control commands for a set of wireless devices, or terminals (e.g. UEs) as a complement to the commands included in the scheduling assignments/grants.        
One PDCCH/EPDCCH carries one DCI message containing one of the groups of information listed above. As multiple wireless devices, or terminals (e.g. UEs) can be scheduled simultaneously, and each terminal can be scheduled on both downlink and uplink simultaneously, there may be a possibility to transmit multiple scheduling messages within each subframe. Each scheduling message is transmitted on separate PDCCH/EPDCCH resources, and consequently there are typically multiple simultaneous PDCCH/EPDCCH transmissions within each subframe in each cell. Furthermore, to support different radio-channel conditions, link adaptation can be used, where the code rate of the PDCCH/EPDCCH is selected by adapting the resource usage for the PDCCH/EPDCCH, to match the radio-channel conditions.
Here follows a discussion on the start symbol for PDSCH and EPDCCH within the subframe. The OFDM symbols in the first slot are numbered from 0 to 6. For transmissions modes 1-9, the starting OFDM symbol in the first slot of the subframe for EPDCCH can be configured by higher layer signaling and the same is used for the corresponding scheduled PDSCH. Both sets have the same EPDCCH starting symbol for these transmission modes. If not configured by higher layers, the start symbol for both PDSCH and EPDCCH is given by the CFI value signaled in PCFICH.
Multiple OFDM starting symbol candidates can be achieved by configuring the UE in transmission mode to, by having multiple EPDCCH Physical Resource Block (PRB) configuration sets where for each set the starting OFDM symbol in the first slot in a subframe for EPDCCH can be configured by higher layers to be a value from {1,2,3,4}, independently for each EPDCCH set. If a set is not higher layer configured to have a fixed start symbol, then the EPDCCH start symbol for this set follows the CFI value received in the Physical Control Format Indicator Channel (PCFICH).
Carrier Aggregation
The 3GPP LTE Rel-10 standard supports bandwidths larger than 20 MHz. One important requirement on LTE Rel-10 is to assure backward compatibility with LTE Rel-8. This should also include spectrum compatibility. That would imply that an LTE Rel-10 carrier, wider than 20 MHz, should appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier can be referred to as a Component Carrier (CC). In particular for early LTE Rel-10 deployments it can be expected that there will be a smaller number of LTE Rel-10-capable terminals compared to many LTE legacy terminals. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e. that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to obtain this would be by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 terminal can receive multiple CC, where the CC have, or at least have the possibility to have, the same structure as a Rel-8 carrier. A CA-capable wireless device (e.g. a UE) is assigned a PCell which is always activated, and one or more SCells which may be activated or deactivated dynamically.
The number of aggregated CC as well as the bandwidth of the individual CC may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same whereas an asymmetric configuration refers to the case that the number of CCs is different. It is important to note that the number of CCs configured in a cell may be different from the number of CCs seen by a terminal. A terminal may for example support more downlink CCs than uplink CCs, even though the cell is configured with the same number of uplink and downlink CCs.
In addition, a key feature of carrier aggregation is the ability to perform cross-carrier scheduling. This mechanism allows a (E)PDCCH on one CC to schedule data transmissions on another CC by means of a 3-bit Carrier Indicator Field (CIF) inserted at the beginning of the (E)PDCCH messages. For data transmissions on a given CC, a UE expects to receive scheduling messages on the (E)PDCCH on just one CC—either the same CC, or a different CC via cross-carrier scheduling. This mapping from (E)PDCCH to PDSCH is also configured semi-statically.
LTE Measurements
A wireless device (e.g. a UE) performs periodic cell search and Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ) measurements in Radio Resource Control (RRC) Connected mode. It is responsible for detecting new neighbor cells, and for tracking and monitoring already detected cells. The detected cells and the associated measurement values are reported to the network or a network node. Reports to the network can be configured to be periodic or aperiodic based a particular event.
Rel-12 LTE Discovery Signal (DRS)
To share the channel in the unlicensed spectrum, the LAA SCell cannot occupy the channel indefinitely. One of the mechanisms for interference avoidance and coordination among small cells is the SCell ON/OFF feature. In Rel-12 LTE, discovery signals were introduced to provide enhanced support for SCell ON/OFF operations. Specifically, these signals are introduced to handled potentially severe interference situation (particularly on the synchronization signals) resulted from dense deployment as well as to reduce UE inter-frequency measurement complexity.
The discovery signals in a DRS occasion are comprised of the Primary Synchronization Signal (PSS), Secondary Synchronization Signal (SSS), Cell-Specific Reference Signal (CRS) and when configured, the channel state information reference signals (CSI-RS). The PSS and SSS are used for coarse synchronization, when needed, and for cell identification. The CRS is used for fine time and frequency estimation and tracking and may also be used for cell validation, i.e., to confirm the cell identity (ID) detected from the PSS and SSS. The CSI-RS is another signal that can be used in dense deployments for cell or transmission point identification. FIG. 2 shows (as differently shaded/striped boxes, see the legend of the figure) the presence of these signals in a DRS occasion of length equal to two subframes in the time dimension (x-axis) (i.e. 2 ms) and also shows the transmission of the signals over two different cells or transmission points (TP1, TP2) for small cell on/off via SCell activation/deactivation.
The DRS occasion corresponding to transmissions from a particular cell may range in duration from one to five subframes for Frequency Division Duplex (FDD) and two to five subframes for Time Division Duplex (TDD). The subframe in which the SSS occurs marks the starting subframe of the DRS occasion. This subframe is either subframe 0 or subframe 5 in both FDD and TDD. In TDD, the PSS appears in subframe 1 and subframe 6 while in FDD the PSS appears in the same subframe as the SSS. The CRS are transmitted in all downlink subframes and Downlink Pilot Time Slot (DwPTS) regions of special subframes.
The discovery signals should be useable by the UE for performing cell identification, reference signal received power (RSRP) and reference signal received quality (RSRQ) measurements. The RSRP measurement definition based on discovery signals is the same as in prior releases of LTE standard specifications. The Received Signal Strength Indicator (RSSI) measurement is defined as an average signal power over all OFDM symbols in the downlink parts of the measured subframes within a DRS occasion (e.g. 1 ms in accordance with Rel-12). The RSRQ is then defined asDRSRQ=N×DRSRP/DRSSI,where N is the number of PRBs used in performing the measurement, DRSRP is the RSRP measurement based on the discovery signals and DRSSI is the RSSI measured over the DRS occasion.
In Rel-12, RSRP measurements based on the CRS and CSI-RS in the DRS occasions and RSRQ measurements based on the CRS in the DRS occasions have been defined. As stated earlier, discovery signals can be used in a small cell deployment where the cells are being turned off and on or in a general deployment where the on/off feature is not being used. For instance, discovery signals could be used to make RSRP measurements on different CSI-RS configurations in the DRS occasion being used within a cell, which enables the detection of different transmission points in a shared cell.
In Rel-13, the RSSI measurement has a duration of from one symbol up to 5 ms, as configured by the eNB.
When measurements are made on the CSI-RS in a DRS occasion, the UE restricts its measurements to a list of candidates sent to the UE by the network via RRC signaling. Each candidate in this list contains a Physical Cell ID (PCID), a Virtual Cell ID (VCID) and a subframe offset indicating the duration (in number of subframes) between the subframe where the UE receives the CSI-RS and the subframe carrying the SSS. This information allows the UE to limit its search. The UE correlates to the received signal candidates indicated by the RRC signal and reports back any CSI-RS RSRP values that have been found to meet some reporting criterion, e.g., exceeding a threshold value.
When a UE is being served on multiple carrier frequencies via a PCell and one or more SCells, the UE needs to perform Radio Resource Management (RRM) measurements on other cells on the currently used carrier frequencies (intra-frequency measurements) as well as on cells on other carrier frequencies (inter-frequency measurements). Since the discovery signals are not transmitted continuously, the UE needs to be informed about the timing of the discovery signals so as to manage its search complexity. Furthermore, when a UE is being served on as many carrier frequencies as it is capable of supporting and inter-frequency RRM measurements need to be performed on a different carrier frequency that is not currently being used, the UE is assigned a measurement gap pattern. This gap pattern on a serving frequency allows the UE to retune its receiver for that frequency to the other frequency on which measurements are being performed. During this gap duration, the UE cannot be scheduled by the eNB on the current serving frequency. Knowledge of the timing of the discovery signals is especially important when the use of such measurement gaps is needed. Beyond mitigating UE complexity, this also ensures that the UE is not unavailable for scheduling for prolonged periods of time on the current serving frequencies (PCell or SCell).
The provision of such timing information is done via a Discovery Measurement Timing Configuration (DMTC) that is signaled to the UE. The DMTC provides a window with a duration of 6 ms occurring with a certain periodicity and timing within which the UE may expect to receive discovery signals. The duration of 6 ms is the same as the measurement gap duration as defined currently in LTE and allows the measurement procedures at the UE for discovery signals to be harmonized regardless of the need for measurement gaps. Only one DMTC is provided per carrier frequency including the current serving frequencies. The UE can expect that the network will transmit discovery signals so that all cells that are intended to be discoverable on a carrier frequency transmit discovery signals within the DMTCs. Furthermore, when measurement gaps are needed, it is expected that the network will ensure sufficient overlap between the configured DMTCs and measurement gaps.
Wireless Local Area Network (WLAN)
In typical deployments of WLAN, carrier sense multiple access with collision avoidance (CSMA/CA) is used for medium access. This means that the channel is sensed to perform a clear channel assessment (CCA), and a transmission is initiated only if the channel is declared as Idle. In case the channel is declared as Busy, the transmission is essentially deferred until the channel is deemed to be Idle. When the range of several Access Points (APs) using the same frequency overlap, this means that all transmissions related to one AP might be deferred in case a transmission on the same frequency to or from another AP which is within range can be detected. Effectively, this means that if several APs are within range, they will have to share the channel in time, and the throughput for the individual APs may be severely degraded. In a listen before talk (LBT) mechanism, used for unlicensed frequency bands as a means for fair access of the band, a device or node wishing to use the band, must first listen to see if it is occupied or not. Before a transmission burst on the LAA SCell, the equipment, device or node (UE or base station) performs a Clear Channel Assessment (CCA) check using “energy detect”. The equipment or node observes the Operating Channel(s) for a defer period and a random number of observation slots. If the channel is found to be idle during these periods, the LBT is declared to have succeeded (LBT success or LBT succeed) and the node can transmit for time duration up to the transmission opportunity (TXOP) duration. The purpose of the defer period is to avoid colliding with Wi-Fi ACK frame transmissions (without LBT) following a Wi-Fi data frame burst. The random number of idle observation slots is designed to randomize the start of transmissions from different nodes that want to access the channel at the same time. Otherwise, if the channel is found to be busy, the LBT is declared to have failed (LBT failure or LBT fails), and no transmission can be made.
Licensed Assisted Access (LAA) to Unlicensed Spectrum Using LTE
Up to now, the spectrum used by LTE is dedicated to LTE. This has the advantage that the LTE system does not need to care about the coexistence issue and the spectrum efficiency can be maximized. However, the spectrum allocated to LTE is limited and cannot meet the ever increasing demand for larger throughput from applications/services. Therefore, a new study item has been initiated in 3GPP on extending LTE to exploit unlicensed spectrum in addition to licensed spectrum. Unlicensed spectrum can, by definition, be simultaneously used by multiple different technologies. Therefore, LTE needs to consider the coexistence issue with other systems such as IEEE 802.11 (Wi-Fi). Operating LTE in the same manner in unlicensed spectrum as in licensed spectrum can seriously degrade the performance of Wi-Fi as Wi-Fi will not transmit once it detects the channel is occupied.
One way to utilize the unlicensed spectrum reliably is to transmit essential control signals and channels on a licensed carrier. That is, aggregation of LTE on licensed and unlicensed frequency bands, which is denoted Licensed assisted access (LAA). AUE is connected to a PCell in the licensed band and one or more SCells in the unlicensed band by using carrier aggregation. In this case we denote a secondary cell in unlicensed spectrum as license assisted secondary cell (LA SCell).
A problem of performing LTE measurements in LAA is that the CRS reference signals are sparsely scheduled and are further subject to LBT.
If a wireless device (e.g. a UE) has been scheduled on a particular subframe on the LAA SCell and tries to perform channel estimation, time-frequency tracking, or decoding when no subframe has actually been transmitted by the SCell, it may severely degrade the accuracy of the tracking loops, RRM measurements, and receiver buffer/soft buffer samples. There is currently no mechanism to prevent the scheduled UEs from attempting to measure a non-existent subframe.
The UE may identify the cell as a declaration of the validity of the measurement. A problem here is that it is difficult for the UE to know if the network did not succeed in the LBT or that it had succeeded but due to a bad channel was not able to identify the cell.